Light emitting diodes have become sufficiently luminous and efficacious in the past several years that they are now considered to be a viable light source for general illumination as well as various specialized illumination applications.
Power LEDs that are suitable for use in illumination applications generally produce a Lambertian-like distribution. (Departures from true Lambertian, can for example take the form a slightly contracted angular range, or somewhat higher intensity at high polar angles.) For many applications other types of light distributions are desired. Examples of other light distribution include, for example, a collimated beam, a beam with a 20° to 30° Full Width at Half Max (FWHM), or a 45° half-angle batwing distribution. In order to alter the light distribution from Lambertian a secondary lens can be used in conjunction with an LED. The lens is termed secondary to distinguish it from the lens that is part of a packaged LED and encapsulates the die of the LED. Examples of secondary lenses that can be used with LEDs to alter the distribution are taught in applicants co-pending, published World Patent Application PCT WO12010/027345.
LEDs and the associated secondary lenses are typically mounted on a printed circuit board (PCB). The LEDs are typically soldered. Soldering provides high quality electrical and thermal connection to the LED. Recently small outline surface mount LED packages that are soldered by the reflow soldering method have become popular. Secondary lenses are typically made of polycarbonate (PC) or polymethylmethacrylate (PMMA) which have a maximum service temperature not more than about 10° C. PC and PMMA are not meant to withstand the temperatures (e.g., 215 to 260 C.) used in reflow soldering. Because of this, the LEDs are first placed (e.g., using a pick-and-place machine) on a circuit board the solder is reflowed and after the LEDs are placed the lenses are attached. Because the LEDs are attached in one process on one apparatus and the lenses are attached in another process, the potential for misalignment of the lens and the LEDs arises. Alignment is important to avoid skewing the light beam emitted by the lens to one side. Aside from the issue misalignment the way in which lenses are attached according to the prior art has certain additional deficiencies.
One prior art way in which lenses for LEDs are mounted is described with reference to FIG. 1. Referring to FIG. 1, an LED 102 is shown mounted on a PCB 104. A lens holder 106 will be mounted on the PCB 104 with glue or an adhesive foam tape (not shown). A collimator type lens 108 will then be snap fit into the holder 106. While power LEDs used in illumination are meant to last tens of thousands of hours, it is unclear that glue or adhesive foam tape attached to the PCB 104 which may operate at temperatures approaching 100 C. would last tens of thousands of hours.
Another prior art way in which lenses for LEDs are mounted is described with reference to FIG. 2. Referring to FIG. 2, an LED 202 is shown mounted on a PCB 204. The PCB 204 has three thru-holes 206. A secondary lens 208 has three integrally molded standoffs 210. The standoffs 210 include end portions 212 which are inserted through the thru-holes 206 and then heat-staked in place. In heat-staking a heated probe is pressed against the end portions 212 and partly crushes them. The heat-staking makes a sort of nail-head shaped portion on the bottom side of the PCB 202. Aside from the complexity of the heat staking operation which is done manually one at a time with a soldering iron fitted with a special tip or one lens at time in a jig, there is also the cost of drilling the holes 206. Another problem with this is approach to attaching the lenses is that the nail head shaped portions formed by crushing the end portion 212 would prevent the PCB 202 from being placed flat on a heat sink (the nail head shaped portions would keep the PCB and heat sink apart.) This is disadvantageous because it is desirable to be simply mount the PCB against the base of a heat sink as a straight forward way to control the temperature of the LEDs.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.